1. Field of the Invention
The present invention relates to a control device of a machine tool which estimates overheating of a motor.
2. Description of the Related Art
When using a spindle of a machine tool for machining, usually it is used at or below a continuous rated load of the spindle motor. The “continuous rated load” is a load of a level where the motor will not overheat even if that load is continuously applied for an unlimited time so long as being used at that load or less. However, when performing heavy duty machining or when increasing the load for just a short time to shorten the processing time, sometimes a load exceeding the continuous rated load is given to operate the motor.
When applying a load exceeding the continuous rated load to a motor, a possible machining time is prescribed in accordance with that load. If continuing to apply a load for a time over this, the motor will end up overheating. Therefore, when applying a load exceeding the continuous rated load to a motor for performing processing, an indicator relating to exactly how much time the processing can be continued without the motor overheating when continuing to apply the current load becomes necessary.
In general, a graph which illustrates an allowable load characteristic which enables information on “how many minutes machining is possible until overheating if machining by a load n %” to be understood for each motor is prescribed for machining exceeding a continuous rated load. The operator performs machining with reference to the illustrated information. In the graph of the allowable load characteristic, the possible machining time corresponding to a load is prescribed. If referring to the graph of the allowable load characteristic, it will be understood how much time it will take for the motor to end up overheating if continuing to apply a certain load.
FIG. 1 is a view which illustrates one example of an allowable load characteristic of a motor. In FIG. 1, the abscissa indicates a motor speed, while the ordinate indicates a load which is applied to the motor. A load of 100% illustrates the continuous rated load. In that case, even if that load is continuously applied for an unlimited time, the motor will not overheat. In the example which is illustrated in FIG. 1, in the low speed rotation region, a 200% load is illustrated as the load which can be applied if machining for 2.5 minutes in a 10 minute cycle, while a 230% load is illustrated as the load which can be applied if machining for 1 minute in a 10 minute cycle. If applying a load exceeding the continuous rated load to make the motor continuously operate, the motor may generate heat exceeding the allowable amount of heat generation and break down.
To deal with this problem, for example, in the method and apparatus for control of a machine tool by numerical control which are disclosed in Japanese Patent Publication No. 2000-271836A, the amount of heat generation of a program as a whole is estimated from the expected values of heat generation of the motor at the time of fast speed of the feed shaft motor (non-machining) and the time of machining speed and the time constant is made to change so that the amount of heat generation becomes the allowable one or less. However, while the invention which is disclosed in Japanese Patent Publication No. 2000-271836A estimates the heat generation of the motor, it does not disclose for exactly how much time machining is possible until overheating.
Further, in a control device of a permanent magnet type synchronous motor which is disclosed in WO2005/093942, the loss which occurs at the motor (copper loss+iron loss) is found, the amount of the rated loss minus that loss is deemed the loss which can be consumed by the motor, and a d-axis current corresponding to this is run so as to prevent overload of the motor. However, in the invention which is disclosed in WO2005/093942, the loss including the iron loss is estimated, there is no disclosure of how much time remains during which machining is possible until overheating.
Therefore, the present applicant proposes preparing a formula for heat simulation which estimates the motor temperature from the value of current of the motor and providing a method of calculating this formula in reverse so as to estimate about how much time remains until the motor overheats, that is, about how much time remains during which continuous machining is possible.
However, in the method which the present applicant proposes, the possible continued machining time from the value of current is estimated, so if the value of current fluctuates slightly due to fluctuation of the machining load, the possible continued machining time will frequently fluctuate and therefore the time will be difficult to grasp.